/Engine/Source/Runtime/Engine/Classes/Engine/Engine.h
This function will add a debug message to the onscreen message list. It will be displayed for FrameCount frames.
void AddOnScreenDebugMessage ( uint64 Key, float TimeToDisplay, [FColor](API\Runtime\Core\Math\FColor) DisplayColor, const [FString](API\Runtime\Core\Containers\FString) & DebugMessage, bool bNewerOnTop, const FVector2D & TextScale )